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No description available.

It is proposed to place a space station in a short period orbit about the L₄ triangular point in the earth moon system for the purpose of communication and possibly astronomical observations. Within limits of a linear analysis, an exhaustive and general approach to the dynamical problem is presented. The equations of motion for a finite sized body of arbitrary shape and internal mass distribution are derived. The phenomenon of resonance and stability are discussed. Stability boundaries for such a space station are established. Initial conditions and pointing errors are analyzed. / Master of Science

As followup to our recent detection of a pre-transit signal around HD 189733 b, we obtained full pre-transit phase coverage of a single planetary transit. The pre-transit signal is again detected in the Balmer lines but with variable strength and timing, suggesting that the bow shock geometry reported in our previous work does not describe the signal from the latest transit. We also demonstrate the use of the Ca II H and K residual core flux as a proxy for the stellar activity level throughout the transit. A moderate trend is found between the pre-transit absorption signal in the 2013 data and the Ca II H flux. This suggests that some of the 2013 pre-transit hydrogen absorption can be attributed to varying stellar activity levels. A very weak correlation is found between the Ca II H core flux and the Balmer line absorption in the 2015 transit, hinting at a smaller contribution from stellar activity compared to the 2013 transit. We simulate how varying stellar activity levels can produce changes in the Balmer line transmission spectra. These simulations show that the strength of the 2013 and 2015 pre-transit signals can be reproduced by stellar variability. If the pre-transit signature is attributed to circumplanetary material, its evolution in time can be described by accretion clumps spiraling toward the star, although this interpretation has serious limitations. Further high-cadence monitoring at H alpha is necessary to distinguish between true absorption by transiting material and short-term variations in the stellar activity level.

Command generation is a process by which input commands are constructed or modified such that the system's response adheres to a set of desired performance specifications. Previously, a variety of command generation techniques such as input shaping have been used to reduce residual vibration, limit transient deflection, conserve fuel or adhere to numerous other performance specifications or performance measures. This dissertation addresses key issues regarding the application of command generation techniques to tethered satellite systems. The three primary objectives of this research are as follows: 1) create analytically commands that will limit the deflection of flexible systems 2) combine command generation and feedback control to reduce the retrieval time of tethered satellites, and 3) develop command generation techniques for spinning tether systems. More specifically, the proposed research addresses six specific aspects of command generation for tethered satellites systems: 1) create command shapers that can limit the trajectory tracking for a mass under PD control to a pre-specified limit in real time 2) create commands analytically that can limit the transient deflection of a model with one rigid-body and one flexible mode during rest-to-rest maneuvers 3) command generation for a 2-D model of earth-pointing tethered satellites without tether flexibility, 4) command generation for a 2-D model of earth-pointing tethered satellites to reduce tether retrieval time and reduce swing angle, 5) command generation for a 3-D model of earth-pointing tethered satellites without tether flexibility, and 6) command generation for improved spin-up of spinning tethered satellite systems. The proposed research is anticipated to advance the state-of-the-art in the field of command generation for tethered satellite systems and will potentially yield improvements in a number of practical satellite and tether applications.

Avec la découverte de milliers de nouvelles planètes au cours des vingt dernières années, une nouvelle population complexe de planètes plus petites que Neptune et plus grandes que la Terre a été découverte. Ces planètes se divisent en deux groupes : les plus grandes sub-Neptunes avec des atmosphères étendues dominées par H, et les plus petites super-Terres qui ont tout au plus des atmosphères minces. Cette division peut être expliquée par une variété de mécanismes, y compris la photoévaporation, la perte de masse alimentée par le noyau, et la formation de gaz pauvres et vides : la population de petites planètes est probablement façonnée par une combinaison de ces mécanismes qui peut dépendre du type stellaire. Dans ce travail, nous décrivons la découverte de deux nouvelles planètes qui sont bien adaptées à l'étude de la nature de la population des petites planètes : Wolf 503 b et LP 791-18 d. Wolf 503 b est une planète de \(2.03^{+0.08}_{-0.07} R_{\oplus}\) orbitant autour de l'étoile brillante (\(J=8.32\) mag), proche (\(D=44.5\) pc) à mouvement propre élevé K3.5V Wolf 503 (EPIC 212779563). Nous confirmons que la signature du transit K2 est planétaire en utilisant à la fois des images d'archives et des images d'optique adaptative à haut contraste de l'observatoire Palomar. Son rayon place Wolf 503b directement entre les populations de super-Terre et de sub-Neptune, un rayon auquel les planètes sont rarement trouvées et la composition de masse attendue est ambiguë, et la luminosité de l'étoile hôte fait de Wolf 503b une cible de choix pour le suivi des vitesses radiales et la spectroscopie de transit. La deuxième planète que nous présentons est une planète de taille terrestre orbitant autour de la naine froide M6 LP 791-18. La nouvelle planète d rejoint un système bien aligné avec au moins deux autres planètes, la plus externe étant une sous-Neptune, offrant une occasion unique à ce jour d'étudier un système avec une planète de taille terrestre tempérée et une sous-Neptune qui a conservé son enveloppe gazeuse ou volatile. La découverte de LP 791-18d permet de mesurer la masse du système grâce aux variations du temps de transit, et nous trouvons une masse de \( {9.3_{-1.4}^{+1.5}\,M_\oplus}\) pour la sub-Neptune LP 791-18c et une masse de \( {0.8_{-0.4}^{+0.5}\,M_\oplus}\) pour l'exo-Terre LP 791-18d (\({<2.3 M_{\oplus}}\) à 3\( {\sigma}\)). La planète est également soumise à un fort réchauffement continu par les marées, ce qui peut entraîner une activité géologique et un dégazage volcanique. Pour l'avenir, LP 791-18d et Wolf 503b offrent des opportunités uniques d'étudier les origines et la conservation des atmosphères des petites planètes. / With the discovery of thousands of new planets in the past twenty years, a new and complex population of planets has been discovered which are smaller than Neptune and larger than the Earth. These planets are split into two groups: the larger sub-Neptunes with extended H-dominated atmospheres, and the smaller super-Earths which have at most thin atmospheres. This division can be explained by a variety of mechanisms, including photoevaporation, core-powered mass-loss, and gas-poor and gas-empty formation: the small-planet population is likely shaped by a combination of these which may depend on stellar type. In this work we describe the discovery of two new planets which are well-suited to investigating the nature of the small planet population: Wolf 503b and LP 791-18d. Wolf 503 b is a \(2.03^{+0.08}_{-0.07} R_{\oplus}\) planet orbiting the bright (\(J=8.32\) mag), nearby (\(D=44.5\) pc) high proper motion K3.5V star Wolf 503 (EPIC 212779563). We confirm that the K2 transit signature is planetary using both archival images and high-contrast adaptive optics images from the Palomar observatory. Its radius places Wolf 503 b directly between the populations of super-Earths and sub-Neptunes, a radius at which planets are rarely found and the expected bulk composition is ambiguous, and the brightness of the host star makes Wolf 503b a prime target for radial velocity follow-up and transit spectroscopy. The second planet we introduce is an Earth-sized planet orbiting the cool M6 dwarf LP 791-18. The new planet d joins a well-aligned system with at least two more planets, the outermost being a sub-Neptune, providing a to-date unique opportunity to investigate a system with a temperate Earth-sized planet and a sub-Neptune that retained its gas or volatile envelope. The discovery of LP 791-18d makes the system amenable to mass measurements via transit timing variations, and we find a mass of \( {9.3_{-1.4}^{+1.5}\,M_\oplus}\) for the sub-Neptune LP 791-18c and a mass of \( {0.8_{-0.4}^{+0.5}\,M_\oplus}\) for the exo-Earth LP 791-18d (\( {<2.3 M_{\oplus}}\) at 3\( {\sigma}\)). The planet is also subject to strong continued tidal heating, which may result in geological activity and volcanic outgassing. Looking forward, LP 791-18d and Wolf 503b offer unique opportunities to study the origins and retention of small-planet atmospheres.

In this dissertation, we present three complementary studies of the processes that drive planetary migration. The first is a radial-velocity survey in search of giant planets in adolescent (<1 >Gyr) open clusters. While several different mechanisms may act to drive giant planets inward, only some mechanisms will excite high eccentricities while doing so. Measuring the eccentricities of young hot Jupiters in these clusters (at a time before the orbits have had a chance to circularize due to tidal friction with their host stars) will allow us to identify which mechanisms are most important. Through this survey, we detect the first 3 hot Jupiters in open clusters (and at least 4 long-period planets), and we measure the occurrence rate of hot Jupiters in clusters to be similar to that of the field (~1%). We determine via analyses of hot Jupiter eccentricities and outer companions in these systems that high eccentricity migration mechanisms (those requiring the presence of a third body) are important for migration. The second project, an adaptive optics imaging survey for stellar companions to known hot Jupiter hosts, aims to determine the role that stellar companions in particular play in giant planet migration. Through a preliminary analysis, we derive a lower limit on the binary frequency of 45% (greater than that of the typical field star), and we find that the presence of a companion is correlated with misalignment of the spin-orbit angle of the planetary system, as would be expected for stellar Kozai-Lidov migration: at least 74% of misaligned systems reside in binaries. We thus conclude that among high eccentricity migration mechanisms, those requiring a stellar companion play a significant role. Finally, we describe simulations of measurements of the planet population expected to be discovered by TESS, and use these to demonstrate that a strong constraint on the obliquity distribution of small planets can be derived using only TESS photometry, Gaia astrometry, and vsin(i) measurements of the host stars. This obliquity distribution will be a key piece of evidence to help detemine the likely formation and migration histories of small planets, and can contribute to the assessment of the potential for Earth-like planets to harbor life.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The telecommunications link between a LEO satellite and a rural ground station with a non-tracking antenna, has a strongly varying link quality and a short communications window. The satellite acts as a store-and-forward node between ground stations. The TC-SDLP and an FTP protocol form a shallow protocol stack, which excludes unneeded protocol functionality and the resulting overhead. Coding gain, introduced by BCH FEC in the TCSDLP, allows for link quality improvement. The core of this thesis is an improvement of the TC-SDLP to maximise effective payload data throughput, or goodput. This improvement was achieved by creating an optimal segment length selection metric based on the BER. Since the BER is not determinable from within the TC-SDLP, the metric was twice determined; once based on the FER and finally based on time delays. The work includes an extensive background study, which consists of space standardisation, orbital physics, error detection and correction, space datalink protocols, data throughput and culminating in the protocol stack design. The project specific link budget calculation is presented. The optimal segment length policy was mathematically determined. Asimulation model of the TC-SDLP was used as a proof of concept for the effective throughput and give a performance benchmark. Finally a TC-SDLP implementation offers a real world performance demonstration. / AFRIKAANSE OPSOMMING: Die telekommunikasie skakel tussen ’n lae aardomwenteling (LEO) sateliet en ’n plattelandse grondstasie met ’n nie-volg antenna, het ’n skakelkwaliteit wat in ’n groot mate varieer en ’n kort kommunikasievenster. Die sateliet tree op as ’n stoor- en- aanstuur node tussen grondstasies. Die TC-SDLP en ’n leêr oordrag protokol (FTP) vorm ’n vlak protokol stapel, wat onnodige protokol funksionaliteit en die gevolglike opkoste uitsluit. Kode aanwins, wat deur die BCH FEC in die TC-SDLP, aangebring word, verbeter die skakelkwaliteit. Die kern van hierdie tesis is ’n verbetering van die TC-SDLP om sodoende die ware deurvoer van nuttige vragdata te maksimimeer. Hierdie verbetering is bereik deur die skep van ’n optimale segmentlengte-seleksie metode gebaseer of die bit fout tempo (BER). Aangesien die BER nie bepaal kan word vanuit die TC-SDLP nie, is die maatstaf twee keer bepaal; die eerste keer is die bepaling gebaseer op die raamwerk fout tempo (FER) en die finale bepaling op tyd vertragings. Die tesis sluit ’n omvattende agtergrondstudie in, wat bestaan uit ruimte standardisering, wentelbaan fisika, die opspoor en regstel van foute, ruimte inligtingskakel protokol en deurstuur van data wat uitloop op die protokol ontwerp. Daar word aangedui hoe die berekening van die begroting vir die skakel van toepassing op die spesifieke projek, gedoen is. ’n Wiskundige analise van die optimale segmentlengte s ook gedoen. ’n Simulasie model van die TC-SDLP is gebruik as ’n bewys van die konsep vir die ware deurset en gee ’n prestasie maatstaf. Laastens bied die TCSDLP implementering ’n ware wereld prestasie demonstrasie.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The use of electrodynamic tethers on-board satellites is an exciting scientific prospect. These conductive tethers provide the means for satellites to generate power and to do propulsion by electrodynamic interaction with the geomagnetic field. Although well researched in theory, the concept has not enjoyed much success in practice. This study aims to utilise low-cost CubeSats as experimental tool to verify many of the theoretical principles that govern the behaviour of conductive tethers in orbit. The study provides a theoretical background of the concept by evaluating past tether missions and analysing existing theory. A feasible application of an electrodynamic tether within the size and weight limitations of a Nano-satellite is formulated. Existing theoretical work is adapted to model the dynamics and electrodynamics of specifically Nano-satellites. Using these mathematical models, control and estimation algorithms are designed which would provide stable deployment of a tethered CubeSat pair and stable control of the orientation of the tethered system. To be able to implement these algorithms on a satellite mission, a prototype of a sensor capable of measuring the angle of the tether using a CMOS camera is designed and built. A hardware platform is built to test the deployment of the tether using an electric motor. Electronics are designed to control the operation of the camera, to do motor control, and to run control and estimation algorithms. Using the results obtained from the practical tests done on the hardware, and using the theoretical models and control algorithms designed, a full orbital simulation of the deployment was done. This simulation includes the performance of the deployment system, the electrodynamic performance of the tether in earth‟s plasmasphere, and the estimation and control algorithms to control the system. Different deployment strategies are analysed and their performance are compared. / AFRIKAANSE OPSOMMING: Die gebruik van elektrodinamiese toue aanboord satelliete is 'n opwindende wetenskaplike vooruitsig. Hierdie geleidende toue verleen aan die satelliete die vermoë om krag op te kan wek en propulsie deur elektriese interaksie met die geomagnetiese veld te kan doen. Alhoewel dit goed nagevors is in teorie, het die konsep nog nie veel sukses in die praktyk geniet nie. Hierdie studie het dit ten doel om lae-koste CubeSats aan te wend as 'n eksperimentele instrument om baie van die teoretiese beginsels wat geld vir die gedrag van geleidende toue in wentelbane te verifieer. Die studie bied 'n teoretiese agtergrond van die konsep deur die evaluering van vorige tou-missies sowel as die analise van bestaande teorie. 'n Uitvoerbare toepassing van 'n elektrodinamiese tou binne die grootte- en gewigsbeperkinge van 'n Nano-satelliet is geformuleer. Bestaande teoretiese werk is aangepas om die dinamika en elektrodinamika spesifiek van toepassing op Nano-satelliete, te modelleer. Deur hierdie wiskundige modelle te gebruik, is beheer- en afskattingsalgoritmes ontwerp wat stabiele ontplooiing van 'n verbinde CubeSat-paar en stabiele beheer van die oriëntasie van die verbinde stelsel sal verseker. Om hierdie algoritmes te implementeer op 'n satelliet-sending, is 'n prototipe van 'n sensor wat in staat is om die hoek van die tou met behulp van 'n CMOS kamera te meet, ontwerp en gebou. 'n Hardeware platform is gebou om die ontplooiing van die tou met behulp van 'n elektriese motor te toets. Elektronika is ontwerp om die kamera te beheer, motor beheer te doen asook om beheer- en afskattingsalgoritmes uit te voer. Deur gebruik te maak van die resultate wat verkry is tydens die praktiese toetse wat gedoen is op die hardeware, en deur gebruik te maak van die teoretiese modelle en beheeralgoritmes wat ontwerp is, is 'n volle wentelbaan-simulasie van die ontplooiing gedoen. Hierdie simulasie sluit die gedrag van die ontplooiingstelsel, die elektriese gedrag van die geleidende tou in die aarde se plasmasfeer, en die afskatting- en beheeralgoritmes om die stelsel te beheer in. Verskillende ontplooiingstrategieë word ontleed en hul gedrag word vergelyk.

Stellar flares are a frequent occurrence on young low-mass stars around which many detected exoplanets orbit. Flares are energetic, impulsive events, and their impact on exoplanetary atmospheres needs to be taken into account when interpreting transit observations. We have developed a model to describe the upper atmosphere of extrasolar giant planets (EGPs) orbiting flaring stars. The model simulates thermal escape from the upper atmospheres of close-in EGPs. Ionisation by solar radiation and electron impact is included and photo-chemical and diffusive transport processes are simulated. This model is used to study the effect of stellar flares from the solar-like G star HD 209458 and the young K star HD 189733 on their respective planets, HD 209458b and HD 189733b. The Sun is used as a proxy for HD 209458, and is an element of Eridani, as a proxy for HD 189733. A hypothetical HD 209458b-like planet orbiting the very active M star AU Microscopii is also simulated. We find that the neutral upper atmosphere of EGPs is not significantly affected by typical flares on HD 209458 and HD 189733. Therefore, stellar flares alone would not cause large enough changes in planetary mass loss to explain the variations in HD 189733b transit depth seen in previous studies, although we show that it may be possible that an extreme stellar proton event could result in the required mass loss. Our simulations do however reveal an enhancement in electron number density in the ionosphere of these planets, the peak of which is located in the layer where stellar X-rays are absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flare levels and enhanced electron densities last from about 3 to 10 h after the onset of the flare, depending on the composition of the ionospheric layer. The strength of the flare and the width of its spectral energy distribution affect the range of altitudes in the ionosphere that see enhancements in ionisation. A large broadband continuum component in the XUV portion of the flaring spectrum in very young flare stars, such as AU Mic, results in a broad range of altitudes a ff ected in planets orbiting this star. Indeed, as well as the X-ray absorption layer, the layer in which EUV photons are absorbed is also strongly enhanced.